Stock-Based Compensation - Accounting for Stock-based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award
Percentage of share-based compensation expense	15.00%
Weighted average grant date fair value of options granted during the period (per share)	$ 1.25	$ 1.64	$ 7.00
Stock-based compensation	$ 3,641	$ 3,845	$ 5,674
Research and development
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	761	1,348	1,851
Research and development \| Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	88	36	71
Research and development \| Equity Incentive Plans
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	673	1,312	1,780
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	2,880	2,497	3,799
General and administrative \| Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	9	20	48
General and administrative \| Equity Incentive Plans
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	$ 2,871	$ 2,477	3,751
Restructuring costs
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation			24
Restructuring costs \| Equity Incentive Plans
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation			$ 24

Represents the percentage of discount on share price treated as expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Amount of noncash expense for share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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